The subject of the present invention is a high sensitivity gamma camera system.
Medical diagnosis is increasingly calling upon medical imaging techniques and, in particular, nuclear medicine. Despite its mediocre intrinsic image qualities (substantial statistical noise and limited resolution), this nuclear imaging technique is irreplaceable since it affords the practitioner, by way of tracers introduced into the organism to be studied, dynamic information about the physiopathology of the organs and of the functions studied.
Currently, nuclear medicine essentially uses cameras of the Anger type or gamma cameras which make it possible to obtain an image directly under plane projection of the distribution of a radioactive substance, or tracer, emitting gamma radiation. The U.S. Pat. No. 3,011,057 describes the principle of operation of such cameras. Schematically, the apparatus can be divided into two parts: the collimator and the detector. The collimator eliminates most of the gamma rays emitted by the radioactive source to be explored, preserving only those which are parallel to a given direction. To do this, the collimator which consists of a material which absorbs gamma radiation, such as lead, is drilled with a large number of openings of very narrow diameter and whose axes define the direction of projection for the formation of the image. Typically, the collimator has a thickness of the order of 53 mm and includes holes with hexagonal cross section of 1.3 mm semi-width.
The collimator therefore makes it possible to obtain an image projected in the direction defined by the openings of the collimator by transforming the chaotic proliferation of the omnidirectional gamma rays emitted by the various points of the source into a monodirectional latent image.
The detector part of the camera placed behind the collimator is composed essentially of a scintillator for converting the gamma rays into photons and of a system of photodetectors for converting the photons emitted by the scintillator into electrical pulses. Thus, the detector part transforms the latent gamma image into a visible image.
In other words, there is, to a first approximation, a one-to-one relation between a line of points of the gamma-emitting object which lie on a same perpendicular to the plane of the collimator and the point of impact of the photon(s) if it exists, emitted by this line of the observed object, on the photo scintillator.
Although numerous enhancements have been made to the detector part of the Anger camera in order to increase its resolution and lessen these distortions and these field inhomogeneities, the collimator itself has advanced little, remaining in principle a gamma radiation absorbing medium drilled with a multitude of holes as has already been indicated. It is however the collimator which limits the performance of current cameras. Thus, with an intrinsic resolution in respect of the scintillator crystal of the order of 4 mm, the resolution with the collimator is now only 10 mm and the output is lessened by a factor 10.sup.-4. The presence of the type of collimator described above moreover imposes a draconian compromise between the two factors consisting of the resolution and the output of the camera. This setup also entails a substantial degradation in resolution with the distance between the source, that is to say the organ to be observed, and the entrance of the camera.